[MemorySSA] Invalidate optimized Defs upon moving them; NFC

We're currently getting this behavior implicitly, since we determine if
a Def's optimization is valid based on the ID of its defining access.
This is incorrect, though I wouldn't be surprised if this was masked in
part by that we're using a WeakVH to track what Defs are optimized to.
(Not to mention that we don't move Defs super often, AFAICT). I'll
submit a patch to fix this shortly.

This also includes a minor refactor to reduce duplication a bit.

No test is included, since like said, this already happens to be our
behavior. I'll add a test for this with my fix to the other bug
mentioned above.

llvm-svn: 340461